1. Field of the Invention
The present invention relates to a contact lens formed from a polymer based on 2,3-dihydroxypropyl methacrylate (GMA) that has high water content and high water balance. The present invention also relates to methods of making and using such contact lenses. The present invention also relates to polymers and hydrogels of GMA and to methods of making and using such polymers.
2. Description of the Related Art
The literature teaches that high water content lenses are susceptible to dimensional changes during wear because they tend to lose a higher percent of water (dehydrate) relative to lenses of lower water content. Further, the ability to regain the lost water (rehydrate) is important to the dimensional stability of a contact lens. If a lens material absorbs water more rapidly, then the lens will more closely return to a water-saturated state during each blink, when the lens is bathed in tear fluid. Therefore, as a lens begins to dehydrate, a characteristic of rapid rehydration is extremely advantageous for maintaining saturation and maximum stability.
Unfortunately, conventional contact lens development either has ignored the effect of rehydration rate upon lenses or has constructed lenses of materials with a less than optimal rate of rehydration.
U.S. Pat. No. 5,532,289, hereby incorporated by reference in its entirety, describes a family of copolymers useful for making contact lenses. The copolymers have improved ability to remain near their water saturated state during wear. In this patent, the inventors of the present invention describe a method for making contact lenses with improved dimensional stability based on copolymers of 2,3-dihydroxypropyl methacrylate (GMA) and 2-hydroxyethyl methacrylate (HEMA). The polymers contain between 20 and 90 mole percent of GMA and have a water content from about 40 to about 70% by weight. The dimensional stability of these lenses results from the ability of these hydrogels to strongly bind water.
In the '289 patent, lenses made from this family of materials were shown, under in-vitro conditions, to be slow to dehydrate and fast to rehydrate, relative to all other soft lens materials. The times it took a lens to dehydrate to 10% of its water weight and rehydrate back to saturation were combined into a factor called "water balance". This "water balance" value can be used to compare commercial lens materials, regardless of water content and as a guideline, by practitioners, in selecting an appropriate lens material for a patient. This patent also emphasizes the ability to remain near or at saturation as a key to dimensional stability in a soft lens.
Clinical studies by Businger in Contact Lens Spectrum, August 1995, pp. 19-25 and die Kontaklinsen 7-8, 4 (1997) support the laboratory findings on water retention and lens stability. These studies report that lenses made from hioxifilcon A (p-GMA/HEMA, 59% water) were preferred by the largest margin of test subjects because of comfort and visual acuity. These lenses showed the highest dimensional stability and the best of both low-contrast and high-contrast visual acuity during wear cycles of 14 hours, over a period of 30 days, over other conventional contact lenses irrespective of water content.
The literature teaches that the water content of GMA/HEMA copolymers increases, from an initial 38% for p-HEMA, with increasing concentration of GMA. See, Yasuda, et. al., Journal of Polymer Science: Part A1, 4, 2913-27 (1966) and Macret et. al., Polymer, 23 (5) 748-753 (1982) , which describes hydrogels based on HEMA and GMA.
Refojo, Journal of Applied Polymer Science, 9, 3161-70 (1965), describes hydrogels of high water content made from GMA. The polymer are made by polymerization in bulk or in water solution. Also, high water content hydrogels of copolymers of GMA and glycidyl methacrylate are described.
Wichterle, et. al., UK Patent GB 2196973A, reported the use of hydrophilic solvents, such as glycerol, dimethylformamide, and dimethylsulfoxide, in 2-HEMA blends primarily for the centrifugal casting of contact lenses.